Method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate

ABSTRACT

A method for producing a titanium nitride coating on the surface of a titanium or titanium alloy substrate may include: a) immersing the titanium or titanium alloy substrate as an electrode in a non-aqueous electrolyte comprising an ionic liquid having nitrogen ions in the presence of a counter electrode; and b) activating an electrochemical process of nitriding the substrate by applying an electric potential between the electrode, and the counter electrode, to generate an anodic electric current to decompose the nitrogen ions by releasing the nitrogen contained therein. The liberated nitrogen penetrates the titanium or titanium alloy substrate until it leads to the conversion to titanium nitride of a surface layer of the substrate, thereby generating a nitrided diffusion surface layer that forms a nitrided surface coating. The electric potential and/or the anodic electric current are modulated in time according to the desired thickness for the nitrided surface coating.

FIELD OF APPLICATION

The subject of this invention is a method for producing a titaniumnitride coating on the surface of a titanium or titanium alloysubstrate.

The method according to the invention finds application in anytechnological field which needs to improve the tribological performanceof components made of titanium or titanium alloys by means of titaniumnitride surface coatings. The application may therefore range fromprecision mechanics to the aerospace industry, from the medical sectorto dental implantology.

Advantageously, the method according to the invention enables titaniumnitride coatings to be produced on substrates made of titanium or itsalloys operating at low temperatures, and in particular at roomtemperature. It therefore finds particular application in producing saidcoatings on parts or components that cannot be heated without losingsome of their geometric features (flatness, roughness, etc.).

The method according to the invention may find particular application inthe automotive industry, for example in the manufacture of protectivesurface coatings for brake system components, such as brake caliperpistons or brake disc parts made of titanium or its alloys.

PRIOR ART

Titanium and its alloys have some very attractive properties that allowthem to be used in many industries. Some of the aforesaid propertiesare: excellent corrosion resistance and erosion resistance; low density,which gives high specific strength-to-weight ratios, allowing forlighter and stronger structures; high temperature resistance; and, insome cases, cryogenic properties.

Titanium and its alloys, however, are also characterized by modesttribological properties, such as poor resistance to abrasive wear, poorresistance to fatigue wear (fretting), and a high friction coefficient.All of this has significantly limited the use of titanium and its alloysin mechanical engineering applications.

The friction problem is related to the crystal structure and reactivityof titanium and may be largely overcome by appropriate thermochemicaltreatments that superficially modify the titanium substrate, making itharder.

One of the most common thermochemical treatments of titanium and itsalloys is nitriding.

To date, the nitriding of titanium and its alloys may only be achievedusing the following techniques: a) plasma-assisted deposition; b)ion-beam deposition; c) laser melting; d) gas-phase deposition; e)cyanide-containing baths.

These nitriding techniques all require:

-   -   high temperatures, between 400 and 1000° C.;    -   long processing times (up to hundreds of hours);    -   complex installations (vacuum systems, high temperature        chambers, etc.).

Not least, the aforesaid techniques generally present significant healthand safety issues. For example, the use of cyanide baths, an extremelytoxic compound, is cited.

As a result, traditional nitriding techniques are not suitable for usein most industrial applications. In particular, it is basicallyimpossible to process large parts, as well as thin parts, as they areeasily subject to thermal deformation.

Particularly problematic is the constraint given by conventionalnitriding techniques on process temperatures. In fact, these techniquesare not applicable on parts or components that may not be heated withoutlosing some of their geometric characteristics.

Ph. Roquiny et al., “Colour control of titanium nitride coatingsproduced by reactive magnetron sputtering at temperature less than 100°C.”, Surface and Coatings Technology 116-119 (1999) 278-283 [1] and S.Bellucci et al., “Synthesis of Titanium Nitride Film by RF Sputtering”,Nanosci. Nanotechnol. Lett. 3 (2011) 1-9 [2] describe methods for thepreparation of titanium nitride films at low temperature using plasmatechnology (reactive magnetron sputtering) in DC or AC mode,respectively. However, this technique has some limitations, namely:

-   -   it requires expensive high-vacuum systems and magnetron        technology for radio frequency control, which seem difficult to        scale to a practical industrial level;    -   it allows only for the treatment of small components that will        fit inside a vacuum chamber;    -   it may not be used to treat components with complex geometry        (presence of shielded areas that cannot be reached by the        plasma); and    -   it suffers from a poor deposition rate with the need for long        treatment times to obtain submillimetric coatings.

CN108752006, CN108946733, and CN108557783 describe the possibility ofusing plasma-based techniques to obtain titanium nitride (TiN)nanopowders at room temperature. However, these documents only refer toobtaining TiN in powder form and do not teach how to make homogeneoussurface coatings. Similarly to the techniques described in [1] and [2],complex vacuum systems are required. In addition, vacuum powder handlingsystems are also required (e.g., CN108752006 envisages a ball millingsystem), making these techniques unsuitable for large batch production.

Thus, there remains a great need for a method to produce a titaniumnitride coating on the surface of a titanium or titanium alloy substratethat may be carried out at low temperatures and is readily applicable onan industrial scale.

In this context, “low temperatures” means temperatures not exceeding250° C., i.e., temperatures that may be considered low when compared tothe operational temperatures of the conventional methods mentioned abovefor the formation of homogeneous surface coatings, which are not lowerthan 400° C.

DISCLOSURE OF THE INVENTION

Therefore, it is a principal object of this invention to eliminate, orat least reduce, the aforementioned problems related to the prior art byproviding a method for producing a titanium nitride coating on thesurface of a titanium or titanium alloy substrate that may be carriedout at low temperatures and is readily applicable on an industrialscale.

A further object of this invention is to provide a method for producinga titanium nitride coating on the surface of a titanium or titaniumalloy substrate that may be carried out at room temperature and isreadily applicable on an industrial scale.

A further object of this invention is to provide a method for producinga titanium nitride coating on the surface of a titanium or titaniumalloy substrate that allows submillimetric coatings, in particular of atleast one micron, to be obtained in a short time.

A further object of this invention is to provide a method for producinga titanium nitride coating on the surface of a titanium or titaniumalloy substrate, which allows a very homogeneous coating to be obtained.

A further object of this invention is to provide a method for producinga titanium nitride coating on the surface of a titanium or titaniumalloy substrate that allows a homogeneous coating to be obtained withoutconditioning by the morphology and dimensions of the substrate.

DESCRIPTION OF THE DRAWINGS

The technical features of the invention are clearly identifiable in thecontent of the claims set out below and the advantages thereof willbecome more readily apparent in the detailed description that follows,made with reference to the accompanying drawings, which represent one ormore embodiments provided purely by way of non-limiting examples,wherein:

FIG. 1 shows a flow diagram of a method for producing a titanium nitridecoating on the surface of a titanium or titanium alloy substrateaccording to a preferred embodiment of the invention;

FIG. 2 shows in two superimposed graphs the time pattern of electricpotential and current density during an electrochemical nitridingprocess according to an example application of the method according tothe invention;

FIGS. 3A and 3B show diffractograms, respectively, of a stoichiometricTiNx coating obtained with the method according to the invention and ofthe comparison thereof with a stoichiometric TiN coating obtained by aconventional plasma technique; and

FIG. 4 shows the polarization curves of a sub-stoichiometric TiNxcoating obtained with the method according to the invention and anon-nitrided Ti6Al4V titanium alloy; and

FIG. 5 shows in two superimposed graphs the time pattern of the electricpotential and current density during an electrochemical nitridingprocess according to a further example of application of the methodaccording to the invention.

DETAILED DESCRIPTION

This invention relates to a method for producing a titanium nitridecoating on the surface of a titanium or titanium alloy substrate.

This method comprises the following operational steps:

-   -   a) immersing the titanium or titanium alloy substrate as an        electrode in a non-aqueous electrolyte consisting of a Room        Temperature Ionic Liquid (RTIL), including nitrogen ions, in the        presence of a counter electrode; and    -   b) activating an electrochemical process for nitriding the        substrate, by applying an electric potential between the        electrode, acting as an anode, and the counter electrode, acting        as a cathode, so as to generate an anodic electric current at        least sufficient to decompose the nitrogen ions by releasing the        nitrogen contained therein.

Operationally, the counter electrode acts as a cathode in the sense thatreduction reactions may occur on its surface.

During the aforesaid electrochemical nitriding process, nitrogenreleased from the decomposition of the nitrogen ions of the ionic liquidpenetrates by diffusion into the titanium or titanium alloy substrate,leading to the conversion of a surface layer of said substrate intotitanium nitride. Thus, a nitrided surface diffusion layer is generatedon the substrate, forming a surface coating for the substrate.

Operationally, said electric potential and/or anodic current aremodulated over time as a function of the thickness desired for theaforesaid nitrided surface coating.

Advantageously, the electrochemical nitriding process is carried out atlow temperatures, i.e., at temperatures below 250° C., preferably below200° C.

The upper temperature limit at which the electrochemical process may becarried out is defined by the thermal stability of the ionic liquid.Once the thermal stability limit is exceeded, the ionic liquid degradescompletely or partially, and the electrochemical nitriding process maynot proceed.

The electrochemical nitriding process is thus carried out attemperatures not exceeding the temperature of maximum thermal stabilityof the ionic liquid.

According to a preferred embodiment of the method, the electrochemicalnitriding process is carried out at room temperature. This isparticularly advantageous as it simplifies the operational management ofsaid process.

From an operational point of view, however, carrying out theelectrochemical nitriding process at low temperatures (not exceeding250° C.) limits the mobility of nitrogen in the crystalline matrix ofthe titanium or titanium alloy, with consequences on the chemicalfeatures of the titanium nitride obtained, as illustrated hereinafter.

Advantageously, said nitrided surface coating that is obtained by themethod according to the invention is composed of sub-stoichiometrictitanium nitride TiNx, wherein 0≤x≤0.3. Such sub-stoichiometric titaniumnitride has a lower degree of crystallinity than the degree ofcrystallinity of stoichiometric titanium nitride TiN.

The titanium nitride obtainable with the conventional plasma techniqueis a stoichiometric (Ti:N equal to 1:1) and crystalline TiN. Incontrast, the titanium nitride obtainable with the method according tothe invention is a sub-stoichiometric TiN (Ti:N equal to 1:0.3 max) andhas a low degree of crystallinity.

The low crystallinity of the sub-stoichiometric TiNx makes this titaniumnitride much more corrosion-resistant than stoichiometric titaniumnitride. The low degree of crystallinity implies, in fact, a morelimited extension of grain boundaries and therefore a reduction ofreactive sites on which oxidation processes may occur.

The maximum ratio Ti:N of 0.3 is related to the low temperatures atwhich the electrochemical process is carried out. Low processtemperatures limit the mobility of nitrogen in the crystalline matrix ofthe titanium or titanium alloy substrate. The lower the temperature, themore difficult it will be to insert nitrogen into the titanium crystallattice. The use of a room temperature ionic liquid as a nitrogensource, instead of a high T gas (as in the conventional plasmatechnique) thus affects the features of the titanium nitride obtained.In other words, the sub-stoichiometry (and thus the superior corrosionresistance) is a direct result of operating at low temperatures.

Advantageously, the nitrided surface coating obtainable by the methodaccording to the invention has an average thickness between 0.040 μm and5 μm. These thicknesses may be obtained by prolonging the aforementionedstep b) of activating an electrochemical nitriding process for a periodof time between 5 and 45 minutes, variable according to the selectedtemperature and the way in which the electrochemical process is carriedout.

According to a preferred embodiment of the invention, as illustrated inthe flow diagram of FIG. 1 , the aforesaid electrochemical processcomprises two consecutive steps:

-   -   a first galvanostatic step, wherein an electric potential is        applied, modulated in time so as to generate an anodic electric        current having a value on average equal to a predefined base        current density, until a predefined threshold electric potential        is reached; and    -   a second potentiostatic step, wherein an electric potential        equal to a predefined base electric potential is applied and        maintained until at least one anodic current with a predefined        threshold current density is reached.

It has been experimentally verified that performing in sequence first agalvanostatic step and then a potentiostatic step maximizes theefficiency of the electrochemical nitriding process in terms of thecoating thickness obtained and the amount of nitrogen inserted into thecrystalline matrix of the titanium or titanium alloy substrate.

More specifically, at the beginning of the electrochemical nitridingprocess (zero coating thickness), it is preferable that the diffusion ofnitrogen into the substrate matrix occurs in a slow and controlledmanner. A galvanostatic step is performed for this purpose. Once acertain amount of TiNx is formed, the electrical resistance of thecoating begins to increase, and in order for nitrogen diffusion in thesubstrate matrix to proceed efficiently, it is necessary to maintain anelectric potential on average equal to a predefined value. Apotentiostatic step is carried out for this purpose.

The electrochemical nitriding process may also be carried out byperforming only a galvanostatic step or only a potentiostatic step.However, being the conditions equal, with respect to the execution ofthe electrochemical process in the aforesaid two consecutive steps, anitrided coating with a lower thickness and associated lower nitrogeninsertion is obtained.

As described above, during the first galvanostatic step, atime-modulated electric potential is applied so as to generate an anodicelectric current on average equal to a predefined base current densityuntil a predefined threshold electric potential is reached.

Preferably, said predefined base current density is between 0.025 and0.5 mA/cm2.

The current density values to be used depend on the electricalresistance of the substrate being treated and how much native oxide ispresent on the surface. If the substrate is not very resistive (with athickness of a few nanometers of native oxide), a base current densityvalue close to 0.025 mA/cm2 may be used; conversely, if the substrate isvery resistive (with many nanometers of native oxide), a base currentdensity value in the vicinity of 0.5 mA/cm2 may be used.

Titanium and titanium alloys have an extremely negative redox potential(−1.63V vs. NHE, normal hydrogen electrode). This means that titanium isalways covered with a thin layer of native oxide and the naturaloxidation of its surfaces is virtually instantaneous. Moreover, thenative titanium oxide is particularly dielectric, i.e., it has adielectric constant higher than many other transition metal oxides;therefore, even very small thicknesses, for example less than 100 nm,are sufficient for the electric field to be very attenuated therein. Asalready pointed out, this behavior forces the use of current densitiesthat may be different depending on the initial conditions of thesubstrate to be treated.

The fact that many titanium compounds (including nitride) are verydielectric explains the reason why very thick coatings cannot beachieved and the titanium nitride coating has excellent corrosionresistance.

Preferably, the aforesaid predefined electrical threshold potential isbetween 2 and 12V, and even more preferably between 4 and 10V, and mostpreferably equal to 5V.

The minimum potential value to be applied is the one necessary to makethe nitrogen ions in the ionic liquid decompose and thus make thenitrogen comprised therein available.

Operationally, the more the electric potential grows and approaches 12V,the faster the growth of TiNx occurs. However, for values of appliedelectric potential greater than 10V, the rate of parasitic reactions(causing gas evolution from the ionic liquid bath) becomesnon-negligible and therefore the efficiency of the process is reduced.In other words, the goal in the galvanostatic step is to use the currentto degrade the nitrogen ions in the ionic liquid and not to evolvegaseous products. Electric potential values not exceeding 10V thereforeensure a good compromise between the speed of growth of the coating andthe columbic efficiency of the process.

As described above, during the second potentiostatic step, an electricpotential on average equal to a predefined base electric potential isapplied and maintained until at least one anodic current having apredefined threshold current density is reached.

Preferably, the aforesaid predefined base electric potential is between8 and 50V, and even more preferably equal to 10V.

The goal of the potentiostatic step is to support the nitrogen insertionreaction in the substrate matrix by overcoming the electrical resistanceof the nitrate coating that is being formed. The electric potential maytherefore exceed the value of 10V to reach values even close to 50V,although it is preferable to maintain values close to 10V to contain theevolution of gaseous products from the ionic liquid.

Preferably, the aforesaid predefined threshold current density isbetween 20 and 80 μA/cm2, and even more preferably 50 μA/cm2.

It has been experimentally verified that below this threshold value theelectrochemical process becomes completely inefficient as there isalmost no growth in coating thickness against a progressive increase inenergy consumption.

Preferably, the aforesaid second potentiostatic step has a duration ofat least 5 minutes, regardless of the value of the threshold currentdensity.

During the aforesaid first galvanostatic phase, the anodic electriccurrent may be maintained on average equal to the predefined basecurrent density in a constant manner (as illustrated for example in FIG.2 ) or in a pulsed pattern (as illustrated for example in FIG. 5 ).

Preferably, in the case of a pulsed pattern, the amplitude of thecurrent density pulses with respect to the mean value is at least ±10%.For smaller amplitudes the pulsed pattern leads to effects substantiallyequivalent to those of the constant pattern.

Preferably, each current density pulse has a duration of at least 100 ms(milliseconds).

During the aforesaid second potentiostatic step, the electric potentialmay be maintained on average equal to the predefined base electricpotential in a constant manner (as illustrated for example in FIG. 2 )or in a pulsed pattern.

Preferably, in the case of a pulsed pattern, the amplitude of theelectric potential pulses with respect to the mean value is at least±10%. For smaller amplitudes the pulsed pattern leads to effectssubstantially equivalent to those of the constant pattern.

Preferably, each electric potential pulse has a duration of at least 100ms (milliseconds).

It was possible to verify that carrying out the galvanostatic stepand/or the potentiostatic step with a pulsed pattern allows a series ofadvantages to be obtained with respect to the case of a constantpattern:

-   -   Limiting the possible formation of undesirable by-products on        the surface of the nitrided workpiece; it is hypothesized that        these by-products could be due to parasitic reactions leading to        degradation of the ionic liquid without an associated nitride        formation;    -   Reducing the evolution of gaseous by-products.    -   Improving local mixing of the ionic liquid by allowing the        surface of the treated substrate to be replenished with new        ions; this helps to increase the efficiency of the process.

Preferably, the galvanostatic step and/or the potentiostatic step arecarried out with pulsed patterns of the electric current density and theelectric potential, respectively.

Advantageously, both direct current and alternating current may beapplied in the electrochemical process.

Preferably, the substrate is immersed in the ionic liquid bath via asupport structure, which has the function of keeping the substrateimmersed and simultaneously supplying current from a current/voltagegenerator. Through this support structure it is also possible to measurethe electric potential and the current flow applied to the substrate inorder to manage the electrochemical nitriding process.

The use of a Room Temperature Ionic Liquid (RTIL) as a non-aqueouselectrolytic bath is an essential feature of the method. No solvents areadded to the ionic liquid. In fact, the ionic liquid acts as both asolvent and a reagent. Such a reaction is referred to in the jargon as a“neat reaction.” In general, room temperature ionic liquids arenon-volatile, non-toxic compounds with high thermal stability and ionicconductivity.

Advantageously, during the electrochemical process the ionic liquid maybe subjected to a forced stirring (e.g., by mixers). Stirring allows thesurface of the treated substrate to be replenished with new ions,helping to increase the efficiency of the process.

Preferably, the aforesaid room temperature ionic liquid comprisesnitrogen anions. In fact, it was possible to verify that by using ionicliquids with nitrogen cations the electrochemical process is triggeredwith great difficulty.

According to a preferred embodiment of the method, the aforesaid roomtemperature ionic liquid comprises:

-   -   pyrrolidinium, imidazolium and/or morpholinium cations and    -   dicyanamide and/or tricyanomethanide anions.

Preferably, the pyrrolidinium, imidazolium, and morpholinium cations arefunctionalized with radical groups chosen from the group consisting of:methyl, ethyl, propyl, and butyl, preferably methyl, ethyl, and propyl.

Particularly preferred are anionic liquids having dicyanamide as thenitrogen anion. In fact, the dicyanamide anion consists of 64% nitrogen.There are no liquid compounds with nitrogen anions having sufficientionic conductivity and a similar nitrogen concentration as dicyanamide.

Advantageously, if ionic liquids with anions other than dicyanamide areused, in order to compensate for the lower amount of available nitrogen,for example, the viscosity, melting temperature, and solvent power ofthe ionic liquid may be modulated in order to increase the ionicconductivity of the ionic liquid and thus increase the columbiumefficiency of the electrochemical process. The “solvent power” of theionic liquid refers to its ability to behave like a solvent and thus,for example, to dissolve other materials. However, for the purposes ofthe electrochemical nitriding process, what is important is for there tobe the highest possible concentration of nitrogen, preferably of theanionic type.

According to a wholly preferred embodiment of the method, the aforesaidroom temperature ionic liquid is chosen from the group comprised of:1-propyl-1-methylpyrrolidinium dicyanamide,1-ethyl-1-methylpyrrolidinium dicyanamide, 1-propyl-1-methylimidazoliumdicyanamide, 1-ethyl-1-methylimidazolium dicyanamide, and1-ethyl-3-methylmorpholinium dicyanamide.

According to an alternative embodiment of the method, the aforesaid roomtemperature ionic liquid comprises bis(trifluoromethylsulfonyl)imide orbis(fluorosulfonyl)imide anions.

In particular, the ionic liquid may be chosen from the group consistingof: Tributylmethylmethylammonium bis(trifluoromethylsulfonyl)immide;Butyltrimethylammonium bis(trifluoromethylsulfonyl)immide; Cholinebis(trifluoromethylsulfonyl)immide; 1-Ethyl-3-methylimidazoliumbis(fluorosulfonyl)immide; 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)immide; 1-Methyl-1-propylpiperidiniumbis(trifluoromethylsulfonyl)immide; 1-Butyl-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)immide; Tributylmethylmethylphosphoniumbis(trifluoromethylsulfonyl)immide; Diethylmethylsulfoniumbis(trifluoromethylsulfonyl)immide.

According to a further alternative, the ionic liquid may be chosen fromthe group consisting of: 1-Ethyl-3-methylimidazolium nitrate; and1-Methyl-1-propylpiperidinium tetrafluoroborate.

As pointed out above, the titanium or titanium alloy substrate isimmersed as an electrode in the room temperature ionic liquid in thepresence of a counter electrode.

Preferably, said counter electrode consists of a body made of graphite,stainless steel, titanium, or aluminum. Even more preferably the counterelectrode is made of graphite.

In fact, these materials have a high electrical conductivity and at thesame time do not degrade in the ionic liquid when an electric potentialis applied.

The aforesaid counter electrode (cathode) may consist of:

-   -   a body immersed in the non-aqueous electrolyte (ionic liquid)        similarly to the substrate (electrode/anode); or    -   the same container of the aforesaid non-aqueous electrolyte        (ionic liquid) within which the substrate (electrode/anode) is        immersed.

Advantageously, as illustrated in the diagram in FIG. 1 , the method maycomprise a pre-treatment step c) of the substrate, to be carried outprior to the aforesaid steps a) and b). Pre-treatment consists ofremoving any traces of grease and/or coolant from the surface of thesubstrate.

Preferably, the pre-treatment for removing any traces of grease and/orlubricant-coolant is carried out by immersing the substrate in a polarsolvent for a predetermined period of time, preferably with theadditional application of ultrasound. In particular for example waterand/or ethanol may be used as a polar solvent. If water is used, asurfactant may be added. Immersion in the polar solvent may be followedby washing with distilled water and subsequent air drying.

The pre-treatment step c), while optional, is nevertheless preferred. Infact, it is preferable that before proceeding to steps a) and b) thesubstrate to be coated exposes an electrically conductive surface. Ifthe surface of the substrate is contaminated with processing residues(e.g., lubricant-coolant), it is possible for islands of lowerelectrical conductivity to be formed on which the nitrided coating growsunevenly.

Advantageously, as illustrated in the diagram in FIG. 1 , the method maycomprise a post-treatment step d) of the substrate to be carried outafter the aforesaid steps a) and b). The post-treatment involvesremoving any residual ionic liquid from the nitrided surface of thesubstrate.

Preferably, the post-treatment for removing any residual ionic liquid iscarried out by immersing the nitrided substrate in a polar solvent for apredefined period of time. In particular for example water and/orethanol may be used as a polar solvent. Immersion may be followed bywashing with distilled water and subsequent air drying.

Example of Application

A sample of titanium alloy Ti6Al4V (Al 6% by weight; V 4%), having anarea of about 8 cm2, was used as the substrate.

The sample was immersed in an ultrasonic bath in ethanol for more than30 seconds. After immersion, the sample was rinsed with distilled waterand allowed to air dry. The purpose of this pre-treatment is to removeany grease and/or traces of lubricant-coolant liquid.

In this case, the method for producing a titanium nitride coating on thesurface of the substrate according to the invention was carried out atroom temperature (25° C.).

A bath consisting of 1-propyl-1-methylpyrrolidinium dicyanamide was usedas the electrolytic bath (room temperature ionic liquid; CAS No.:327022-60-6; C₁₃H₂₀N4, the structural formula of which is providedbelow).

After undergoing pre-treatment, the sample was mounted on a supportstructure (rack) configured to measure both the electric potential andcurrent flow of the sample during the electrochemical nitriding process(step (b)), which will be described hereinafter. Thus mounted, thesample was immersed in the electrolytic bath and used as the workingelectrode (anode). A graphite counter electrode (cathode) was alsoimmersed in the bath.

After being immersed in the electrolytic bath (step a)), the sample wassubjected to an electrochemical nitriding process (step b)).

The electrochemical nitriding process consists of two consecutiveelectrochemical steps: the first is a galvanostatic step, while thesecond is a potentiostatic step.

As shown in the graph in FIG. 2 , during the galvanostatic step, ananodic current having a density of 0.025 mA/cm2 was applied to theTi6Al4V sample until an electric potential of 5V was reached. This firstgalvanostatic step lasted approximately 25 minutes. Subsequently, duringthe potentiostatic step, the electric potential of the rack (and thus ofthe sample) was kept constant at 10V until a current of 50 μA/cm2 wasmeasured. This second galvanostatic step lasted approximately 15minutes.

After the electrochemical nitriding process was completed, the samplewas immersed in an ethanol bath for more than 30 seconds. Afterimmersion, the sample was rinsed with distilled water and allowed to airdry.

The sample is found to be coated with a homogeneous layer ofsub-stoichiometric titanium nitride TiNx with x=0.3. The coating had anaverage thickness of about 1 μm and had a typical bright golden color.

FIG. 3A shows the diffractogram of the sub-stoichiometric TiNx coatingobtained on the Ti6Al4V titanium alloy sample. FIG. 3B shows themeasurement of a stoichiometric TiN sample for comparison. It may beobserved that the coating obtained by the method according to theinvention comprises a TiNx layer with x=0.3. With respect to acrystalline TiN (which may be obtained using conventional methods suchas plasma techniques), the pattern of the coating obtained appearsclearly different, therefore indicating a lower degree of crystallinityand a sub-stoichiometric composition.

As a first approximation, the degree of crystallinity may be assessed bylooking at the width of the diffraction peaks. The peaks of the coatingobtained according to the invention TiN_(0.3) show a larger mid-heightwidth with respect to the crystalline TiN.

The value of x may be determined by comparing the measured diffractogramwith that of materials comprised in appropriate databases.

The TiNx coating thus obtained was subjected to line scan voltammetrymeasurements to calculate the corrosion potential and corrosion current.The results are shown in FIG. 4 where the polarization curves of theTiNx (top curve) and the non-nitrided Ti6Al4V alloy sample (bottomcurve) are shown. The comparison of the two curves demonstrates theexcellent corrosion protection capability of TiNx with 0≤x≤0.3. In fact,for the TiN0.3 coating, a corrosion potential of +104 mV vs SCE(Saturated Calomel Electrode) and a corrosion current of 8 nA cm-2 weredetected; the non-nitrided Ti6Al4V alloy sample, on the other hand, hada corrosion potential of −297 mV vs SCE and a corrosion current of 39 nAcm-2.

These two values attest to the impressive corrosion resistance of theTiNx coating, which is higher than that of the Ti6Al4V alloy, one of themost commonly used titanium alloys also due to its excellent corrosionresistance.

Some physicochemical properties of the TiNx coating have been measuredand are shown in Table 1 below:

TABLE 1 Molecular formula TiN_(0.3) Degree of crystallinity low Averagethickness 1 μm Friction coefficient 0.23 Hardness 510 HV Elastic modulus150 GPa Corrosion potential +104 mV vs. SCE Corrosion current 8 × 10 − 9A/cm2

It is also an object of this invention to provide an item, comprising atleast one titanium or titanium alloy portion, said portion having anitrided surface coating consisting of sub-stoichiometric titaniumnitride TiNx, wherein 0≤x≤0.3. Sub-stoichiometric titanium nitride has alower degree of crystallinity than stoichiometric titanium nitride TiN.The aforesaid surface coating is integrated into the crystalline matrixof the titanium or titanium alloy portion.

Preferably, said nitrided surface coating has an average thicknessbetween 0.040 and 5 μm.

Advantageously, the nitrided surface of the aforesaid article isobtained by subjecting the titanium or titanium alloy portion tonitriding by applying the method according to the invention, and inparticular as described above.

The invention allows numerous advantages to be obtained which have beenexplained throughout the description.

The method for producing a titanium nitride coating on the surface of atitanium or titanium alloy substrate according to the invention may becarried out at low temperatures and is readily applicable on anindustrial scale.

In particular, the method according to the invention may be carried outat room temperature in a manner that is easily applicable on anindustrial scale.

The method for producing a titanium nitride coating on the surface of atitanium or titanium alloy substrate according to the invention allowsfor sub-stoichiometric titanium nitride coatings of at least one micronthickness to be obtained in a short time (on the order of a few tens ofminutes).

The method according to the invention also provides a homogeneoustitanium nitride coating.

The method for producing a titanium nitride coating on the surface of atitanium or titanium alloy substrate according to the invention enablesa homogeneous titanium nitride coating to be obtained withoutconditioning by the morphology and dimensions of the substrate to becoated.

With respect to the conventional techniques for generating titaniumnitride coatings, the method according to the invention:

-   -   is based on an electrochemical nitriding process that involves        the simple immersion of the substrate to be treated in an        electrolytic bath, with economic and easy-to-implement modes;    -   allows for large components to be treated without the constraint        of mounting a vacuum chamber;    -   allows for treating any complex geometry that may be immersed in        an electrolytic bath;    -   requires short process times, allowed by the adoption of an        electrochemical process in contrast to the long times of the        traditional plasma technique;    -   is based on a surface conversion of the substrate rather than a        coating deposition, thus avoiding adhesion problems between the        substrate and the surface layer;    -   avoids the use of very toxic reagents, such as cyanide baths.

The invention thus conceived therefore achieves its intended purposes.

Of course, in its practical embodiment it may also assume differentforms and configurations from the one illustrated above, without therebydeparting from the present scope of protection.

Furthermore, all details may be replaced with technically equivalentelements, and dimensions, shapes, and materials used may be anyaccording to the needs.

1-27. (canceled)
 28. A method for producing a titanium nitride coatingon the surface of a titanium or titanium alloy substrate, comprising thefollowing steps: a) immersing the titanium or titanium alloy substrateas an electrode in a non-aqueous electrolyte consisting of an ionicliquid at room temperature, comprising nitrogen ions, in the presence ofa counter electrode; and b) activating an electrochemical nitridingprocess of the substrate, by applying an electric potential between anelectrode, acting as an anode, and the counter electrode, acting as acathode, so as to generate an anodic electric current at leastsufficient to decompose the nitrogen ions releasing the nitrogencontained therein, the released nitrogen penetrating by diffusion intothe titanium or titanium alloy substrate until leading to the conversionof a surface layer of said substrate into titanium nitride, therebygenerating a nitrided diffusion surface layer constituting a nitridedsurface coating for the substrate, wherein said electric potentialand/or said anodic electric current are modulated in time as a functionof the thickness to be obtained for said nitrided surface coating. 29.The method according to claim 28, wherein said non-aqueous electrolyteis at a temperature of less than 250° C., preferably less than 200° C.,and even more preferably at room temperature.
 30. The method accordingto claim 28, wherein said nitrided surface coating consists ofsub-stoichiometric TiNx titanium nitride, where 0≤x≤0.3, saidsub-stoichiometric titanium nitride having a degree of crystallinitylower than the degree of crystallinity of stoichiometric TiN titaniumnitride.
 31. The method according to claim 28, wherein said nitridedsurface coating has an average thickness between 0.040 and 5 μmobtainable by prolonging said step b) of activating an electrochemicalnitriding process for a period of time between 5 and 45 minutes.
 32. Themethod according to claim 28, wherein said electrochemical processcomprises two consecutive steps: a first galvanostatic step, in which anelectric potential is applied which is modulated in time so as togenerate an anodic electric current having a value on average equal to apredefined base current density, until a predefined threshold electricpotential is reached; and a second potentiostatic step, in which anelectric potential is applied on average equal to a predefined baseelectric potential, maintaining it until at least reaching an anodiccurrent having a predefined threshold current density.
 33. The methodaccording to claim 32, wherein during said first galvanostatic step theanodic electric current is maintained on average equal to saidpredefined base current density in a constant manner or in a pulsedpattern.
 34. The method according to claim 32, wherein during saidsecond potentiostatic step the electric potential is maintained onaverage equal to said predefined base electric potential in a constantmanner or in a pulsed pattern, preferably in a pulsed pattern.
 35. Themethod according to claim 32, wherein said predefined base currentdensity is between 0.025 to 0.5 mA/cm2.
 36. The method according toclaim 32, wherein said predefined threshold electric potential isbetween 2 and 12V, and preferably between 4 and 10V, even morepreferably equal to 5V.
 37. The method according to claim 32, whereinsaid predefined base electric potential is between 8 and 50V, and ispreferably 10V.
 38. The method according to claim 32, wherein saidpredefined threshold current density is between 20 and 80 μA/cm2, andpreferably 50 μA/cm2.
 39. The method according to claim 32, wherein saidsecond potentiostatic step has a duration of at least 5 minutes,regardless of the threshold current density value.
 40. The methodaccording to claim 28, wherein said ionic liquid at room temperaturecomprises nitrogen anions.
 41. The method according to claim 28, whereinsaid ionic liquid at room temperature consists of pyrrolidinium,imidazolium and/or morpholinium cations and dicyanamide and/ortricyanomethanide anions, wherein preferably the pyrrolidinium,imidazolium and morpholinium cations are functionalized with radicalgroups chosen from the group consisting of: methyl, ethyl, propyl andbutyl, preferably methyl, ethyl and propyl.
 42. The method according toclaim 41, wherein said ionic liquid at room temperature is selected fromthe group consisting of 1-propyl-1-methylpyrrolidinium dicyanamide,1-ethyl-1-methylpyrrolidinium dicyanamide, 1-propyl-1-methylimidazoliumdicyanamide, 1-ethyl-1-methylimidazolium dicyanamide and1-Ethyl-3-methylmorpholinium dicyanamide.
 43. The method according toclaim 28, wherein said ionic liquid at room temperature comprisesbis(trifluoromethylsulfonyl)imide or bis(fluorosulfonyl)imide anions.44. The method according to claim 28, wherein said ionic liquid at roomtemperature is selected from the group consisting of:Tributylmethylammonium bis(trifluoromethylsulfonyl)immide;Butyltrimethylammonium bis(trifluoromethylsulfonyl)immide; Cholinebis(trifluoromethylsulfonyl)immide; 1-Ethyl-3-methylimidazoliumbis(fluorosulfonyl)immide; 1-Ethyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)immide; 1-Methyl-1-propylpiperidiniumbis(trifluoromethylsulfonyl)immide; 1-Butyl-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)immide; Tributylmethylphosphoniumbis(trifluoromethylsulfonyl)immide; Diethylmethylsulfoniumbis(trifluoromethylsulfonyl)immide.
 45. The method according to claim28, wherein said ionic liquid at room temperature is selected from thegroup consisting of: 1-Ethyl-3-methylimidazolium nitrate; and1-Methyl-1-propylpiperidinium tetrafluoroborate.
 46. The methodaccording to claim 28, wherein said counter electrode consists of a bodymade of graphite, stainless steel, titanium or aluminum and preferablygraphite.
 47. The method according to claim 28, wherein said counterelectrode consists of a body immersed in said non-aqueous electrolyte orthe container of said non-aqueous electrolyte.
 48. The method accordingto claim 28, comprising a pre-treatment step c) of the substrate, to becarried out before said steps a) and b), wherein said pre-treatmentconsists of removing any traces of grease and/or lubricant-coolantliquid from the surface of the substrate.
 49. The method according toclaim 48, wherein said pre-treatment of removing any traces of greaseand/or lubricating coolant liquid is carried out by immersing thesubstrate in a polar solvent for a predefined period of time, preferablywith the additional application of ultrasound, preferably said immersionbeing followed by washing with distilled water and subsequent airdrying.
 50. The method according to claim 28, comprising apost-treatment step d) of the substrate, to be carried out after saidsteps a) and b), wherein said post-treatment consists of removing anyresidues of ionic liquid from the nitrided surface of the substrate. 51.The method according to claim 50, wherein said post-treatment ofremoving any residues of ionic liquid is carried out by immersing thesubstrate in a polar solvent for a predefined period of time, preferablysaid immersion being followed by washing with distilled water andsubsequent air drying.
 52. An article, comprising at least a portion intitanium or titanium alloy, said portion having a nitrided surfacecoating consisting of sub-stoichiometric TiNx titanium nitride, where0≤x≤0.3, said sub-stoichiometric titanium nitride having a degree ofcrystallinity less than the degree of crystallinity of stoichiometricTiN titanium nitride, wherein said surface coating is integrated intothe crystalline matrix of said portion in titanium or titanium alloy.53. The article according to claim 52, wherein said nitrided surfacecoating has an average thickness of between 0.040 and 5 μm.
 54. Thearticle according to claim 52, wherein said nitrided surface is obtainedby subjecting said portion in titanium or titanium alloy to nitriding bya) immersing the titanium or titanium alloy substrate as an electrode ina non-aqueous electrolyte consisting of an ionic liquid at roomtemperature, comprising nitrogen ions, in the presence of a counterelectrode; and b) activating an electrochemical nitriding process of thesubstrate, by applying an electric potential between an electrode,acting as an anode, and the counter electrode, acting as a cathode, soas to generate an anodic electric current at least sufficient todecompose the nitrogen ions releasing the nitrogen contained therein,the released nitrogen penetrating by diffusion into the titanium ortitanium alloy substrate until leading to the conversion of a surfacelayer of said substrate into titanium nitride, thereby generating anitrided diffusion surface layer constituting a nitrided surface coatingfor the substrate, wherein said electric potential and/or said anodicelectric current are modulated in time as a function of the thickness tobe obtained for said nitrided surface coating.